The present invention relates to a machine tool with a revolving spindle borne in a spindle tube which has a receiving or locating opening to receive tool holders, a tool magazine to hold a number of tool holders, a tool changer with at least two gripper arms to transport tool holders between a respective transfer position in the area of the tool magazine and a common spindle position beneath the locating opening and a holding device bearing the tool changer, which device is borne on the spindle tube in such a way that it can be moved longitudinally in the sense of a stroke movement and which transfers the respective tool holder from the spindle position into its working position in the locating opening.
Such a machine tool is known from DE-A-35 21 009.
In the known machine tool the tool magazine is in the shape of a horseshoe. Each of the two gripper arms is assigned its own transfer position from one or the other end of the horseshoe. The two gripper arms are arranged on different sides of the spindle tube which is located between the ends of the magazine's limbs.
The holding device for the two gripper arms is a sleeve located on the spindle tube and which can be moved in a longitudinal direction and whose stroke movement shifts the tool holder, which is located beneath the spindle, between its spindle position and its working position and at the same time moves the tool holder, which is in the transfer position, either upwards or downwards and thus enables its transfer into or removal from the magazine.
As already mentioned, the two gripper arms work on opposite sides of the spindle tube in such a way that if the tool holder of one of the gripper arms is in the lower spindle position the other gripper arm is swivelled laterally in such a way that its tool holder is in the transfer position. The positions of the tool holders held by the gripper arms can be exchanged by swivelling the gripper arms with hydraulic or pneumatic piston-cylinder units so that the other tool holder is now in the common spindle position whereas the first tool holder is in its assigned transfer position.
A tool change can be carried out very quickly with this machine tool but such a machine has to be built very wide such that it projects comparably on either side of the spindle tube since enough space has to be left to the left and right of the headstock for the swivel movement of each arm.
However, it is often desirable for a machine tool to have a number of spindle tubes arranged alongside one another in a minimum of space. In the case of the known machine tool the minimum distance between two spindle tubes would then have to be twice the maximum lateral swivel width of a gripper arm. A double-spindle machine, for example, would thus have to be built very wide.
In order to solve this problem a machine tool is already known from DE-A-38 31 869 in which two gripper arms and a tool magazine are also provided, though the two gripper arms each serve a common transfer position. The two gripper arms are thus arranged on opposite sides of the spindle tube and are :swivelled in such a way that they cross over and avoid contact with one another along complicated locus curves which meet at the end points, resulting in not only a common spindle position but also a common transfer position.
This machine tool is also suitable for double spindle machines since it can be built with a small overall width. The locus curves are guided via a crank element which can be moved longitudinally and which acts upon the gripper arms via two ball-and-socket-joint mechanisms.
The disadvantage of this machine tool is that the gripper arms and their drives are of a very complicated design and that a special transfer device is required to transfer the tool holder between the gripper arm in the transfer position and a corresponding gripper in the tool magazine on account of the complicated locus curves.
Although this machine tool has proven its worth in operation, its construction is nevertheless regarded as very complicated.